Effect of CO2 Pressure on the Rate of Decomposition of Calcite

Article abstract of DOI:10.1021/j150626a004

Rates of CO2 escape from (10<*>1) cleavage surfaces of calcite (CaCO3) single crystals were measured at temperatures from 893 to 1073 K and at CO2 background pressures, P_CO2, from L = 10^-5P_eq to 4 * 10^-2P_eq, where P_eq is the equilibrium decomposition pressure and P_L is the CO2 pressure that produces a flux equal to the flux from a calcite crystal under vacuum.Rates are relatively sensitive to P_CO2 when P_CO2 > 10^-2P_eq, but do not obey the parabolic rate law deduced by earlier investigators from data in a limited pressure range.For P_L < P_CO2 < 10^-2P_eq, the decomposition rates are essentially independent of P_CO2, which implies that in this range the apparent activation enthalpy and entropy are unchanged from those measured under vacuum, ΔH^* = 209 +/- 12 kJ (50 +/- 3 kcal) and ΔS^* = 84 +/- 12 J/deg (20 +/- 3 cal/deg).The rate-limiting process in this pressure range is probably condensed-phase diffusion of CO2 or a surface step for CO2 prior to desorption.